1. Field of the Invention
The present invention generally relates to pumps for delivering pulses of high pressure fluid. More particularly, the invention relates to an improved pump for supplying high-pressure diesel fuel for injection into an internal combustion engine. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Related Art
Fuel pumps for supplying fuel to internal combustion engines are well known in the art. Conventional fuel pumps of this type are, for example, employed by diesel engines. Such diesel pumps are typically rotary pumps which have solid, cylindrical plungers radially reciprocating in corresponding pumping plunger bores. Fuel at inlet pressures is supplied through inlet passages to the plunger bores, and fuel at outlet pressures is discharged through discharge passages from the plunger bores as a result of actuation of the plungers. In the case of a pump which has such radially pressurizing plungers, one or more cam rings provide camming surfaces for actuating the plungers via a plurality of intermediate members. Each intermediate member is typically comprised of a roller which is guided along one or more camming surfaces of one of the cam rings and an adjacent shoe which contacts one of the plungers. Thus, the camming surface of the cam ring slides against the rollers to periodically impart motion to the intermediate members, which motion causes corresponding movement of the plungers within the plunger bores. Although generally effective, pumps of this type suffer from the deficiency that high torque loads and high frictional forces are generated during the sliding of the one-piece plungers within the plunger bores.
Undesirable torque loads (or side-loading) and frictional forces present in conventional pumping plungers during operation thereof arise, in part, from the use of the rotating cam rings to impart radially reciprocal movement of the pumping plungers. In particular, difficulty arises because the cam surface of the rotating cam ring necessarily imparts both radial and tangential force components to the pumping plunger via the intermediate member. While the radial force component produces the desired radial reciprocal movement of the plunger, the tangential force component inherently causes the plungers to cant off-center during motion of the plunger within the bore. This plunger-canting causes the opposite sides of each plunger to bear against the plunger bore during movement of the plunger.
An additional factor which can result in undesirable friction forces being created is plunger deformation during pump operation. As the plungers of conventional pumps experience reciprocal movement, they are subjected to forces on opposite ends thereof. In particular, while one end of the plunger is subjected to the force created by the back-pressure of the fuel to be pumped, the opposite end of the plunger is subjected to the force imparted by the cam ring and intermediate member. In a high pressure fuel pump for a diesel engine, the pressure acting on the plunger can easily be on the order of 20,000 psi (1380 Bar) during normal operation. Accordingly, such plungers typically experience heavy loading which has a tendency to temporarily shorten the length of a plunger and temporarily increase the thickness of the plunger in a direction transverse to the plunger axis. This is especially true in the low pressure section where the plunger is allowed to expand freely. In the high pressure section, the diametral expansion is reduced because of the radial forces generated by the high pressure. This temporary plunger deformation, in turn, exacerbates the frictional loading occurring during plunger movement because a deformed plunger can seize against the wall of a plunger bore.
A final and competing consideration affecting the movement of pumping plungers within the pumping bores is that of fuel leakage through the component interfaces of the pump, e.g., at the interface of the pumping plungers and walls defining the plunger bores. This consideration is important because the seal at the interface of the plungers and the bores must be maintained as tight as possible to maximize pumping pressure and minimize fuel leak-off during operation of the pump. However, component tolerances must still remain loose enough to accommodate the pumping plunger deformation which occurs during operation of the pump. This consideration places a limit on the obtainable quality of the seal between the various components. Thus, while fuel leak-off is desirably minimized, it must be tolerated.
Together, the above-described factors result in high-pressure fuel pumps which experience unusually large fuel leak-off during operation and/or plungers which do not freely travel within the plunger bores during reciprocation thereof.
Accordingly, there remains a need for an improved fuel pump which overcomes the aforementioned deficiencies, by providing an arrangement of pumping plungers which more closely approximate ideal operation during usage.
Further, there remains a need in the art for an improved fuel pump which overcomes the aforementioned deficiencies by reducing fuel leak-off created during operation of the fuel pump.
There also remains a need in the art for an improved fuel pump which overcomes the aforementioned deficiencies by providing an arrangement of pumping plungers which will not seize against the walls of the plunger bores during reciprocation.